I switch to using nitrogen about a year ago.  We use two different
solder finishes on our boards HASL and ENIG.  Have the same problem as
you explain.  We didn't have a problem with tombstones until we stared
to use nitrogen.  The benefit of N2 with increase wetting is what causes
the problem.  I changed our stencils apertures for 0402's to home plate
apertures.  This has a lot but we still have some occurrences.  I have
also changed the land pattern and reduced the size of the pad, this has
helped a lot.  The only thing left that I am going to try is adjusting
my PPM level of O2 to around 900ppm.  This is supposed to help with
tombstoning but retain the benefits of N2.

Hope this helps.

Scott Lefebvre

Scott Lefebvre 


-----Original Message-----
From: Gabriela Bogdan [mailto:[log in to unmask]] 
Sent: Tuesday, October 17, 2006 1:53 PM
Subject: Tombstoning and nitrogen

Netters,
We recently started to use reflow under nitrogen on OSP coated boards.
These boards were previously immersion silver plated and we reflowed
them in air. Very few tombstoning was seen.
Our customer decided to change the finish and requires mandatory reflow
under nitrogen.
We are completing all the soldering operations within 2 days. We use no
clean solder paste and no clean flux.
Questions:
1. Did anybody experience increase of tombstoning of 0402 passives when
using nitrogen?
2. Is using nitrogen during reflow  beneficial for the next process-
selective wave soldering under nitrogen?
3. For thick multilayer boards with more than 20 layers, can complete
hole fill be expected during wave soldering of OSP boards after double
reflow?
If you had any similar experience, could you tell me how you solved
these problems?
Thank you,
Gaby

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